The present invention relates to the purification of propionic acid. More particularly, it relates to a method for the purification of a stream of propionic acid containing water and iodine as impurities.
Propionic acid can be prepared by the reaction of ethylene, water and carbon monoxide or of ethanol and carbon monoxide using catalyst systems formed on mixing of a rhodium or iridium component and an iodine component in the presence of carbon monoxide. See, for example, U.S. Pat. Nos. 3,579,551, 3,579,552, 3,769,329 and 3,772,380. Hydrogen iodide or an alkyl iodide, namely, ethyl iodide, is usually employed as the iodine component in these catalyst systems. While the acid produced by this process is generally of high purity, it sometimes contains residual amounts of iodine either as ionic iodine, free iodine or as ethyl iodide. Such contaminants may render the acid unfit for some uses. In some instances, it may become desirable to produce ultra-pure propionic acid, i.e., acid having an iodine content, as measured by ethyl iodide and ionic iodide (I.sup.-), below a level of 20 parts per billion parts of acid (20 ppb). Distillation techniques are known by which such ultra-pure acids can be obtained but these generally require two or more columns. An example of one such type of distillation is to be found in U.S. Pat. No. 3,772,156, for example, wherein a process for producing an acetic acid product containing less than 40 ppb of iodine is described and claimed. It is an object of the present invention to provide a one-column distillation system wherein crude propionic acid can be converted to ultra-pure propionic acid. Additionally, it is an object of the invention to provide a process to produce ultra-pure propionic acid from ethylene or ethanol without a single waste stream requiring treatment or disposal. Other objects and advantages of the invention will become apparent from the following description thereof.